The present invention relates to transmitters for use in process control loops. More specifically, the present invention relates to isolating transmitter circuitry from the control loop.
Process control systems are used in manufacturing plants to monitor operation of a process. A transmitter is placed in the field and monitors a variable of the process, for example, pressure, temperature or flow. The transmitter couples to a control loop and transmits information over the control loop to a controller which monitors operation of the process. The control loop is a two-wire loop carrying a current which provides power for operation of the transmitter. Communication is through a fieldbus standard which is a digital communications standard in which more than one transmitter can be coupled to a single control loop to transmit the sensed process variable to the control room. This standard is described in ISA 50.02-1992 Section 11. Hart.RTM. is another standard which allows digital communication over a 4-20 mA process variable signal.
Circuitry in the transmitter presents an effective capacitance C.sub.eff to the control loop. Charge may build up on this capacitance causing the transmitter to store energy across its connections to the loop. This energy may discharge through the loop. In the fieldbus protocol, multiple transmitters are attached to the same loop so that the combined C.sub.eff may be particularly large. Attempts have been made to reduce the size of C.sub.eff and thereby reduce the amount of potential energy storage capacity of the transmitter. Additionally, C.sub.eff varies between transmitters. Thus, the interchange of transmitters is limited.
It would be desirable to isolate the capacitance of the transmitter from the control loop. This would prevent energy discharge into the loop and provide greater interchangeability between transmitters regardless of their internal C.sub.eff.